The present invention relates to medical devices. More particularly, the invention relates to a removable vena cava clot filter that can be percutaneously placed in and removed from the vena cava of a patient.
Filtering devices that are percutaneously placed in the vena cava have been available for over thirty years. A need for filtering devices arises in trauma patients, orthopedic surgery patients, neurosurgery patients, or in patients having medical conditions requiring bed rest or non-movement. During such medical conditions, the need for filtering devices arises due to the likelihood of thrombosis in the peripheral vasculature of patients wherein thrombi break away from the vessel wall, risking downstream embolism or embolization. For example, depending on the size, such thrombi pose a serious risk of pulmonary embolism wherein blood clots migrate from the peripheral vasculature through the heart and into the lungs.
A filtering device can be deployed in the vena cava of a patient when, for example, anticoagulant therapy is contraindicated or has failed. Typically, filtering devices are permanent implants, each of which remains implanted in the patient for life, even though the condition or medical problem that required the device has passed. In more recent years, filters have been used or considered in preoperative patients and in patients predisposed to thrombosis which places the patient at risk for pulmonary embolism.
The benefits of a vena cava filter have been well established, but improvements may be made. For example, filters generally have not been considered removable from a patient due to the likelihood of endotheliosis of the filter during treatment. After deployment of a filter in a patient, proliferating intimal cells begin to accumulate around the filter struts which contact the wall of the vessel. After a length of time, such ingrowth prevents removal of the filter without risk of trauma, requiring the filter to remain in the patient. As a result, there has been a need for an effective filter that can be removed after the underlying medical condition has passed.
Moreover, conventional filters commonly become off-centered or tilted with respect to the hub of the filter and the longitudinal axis of the vessel in which it has been inserted. As a result, the filter including the hub and the retrieval hook engage the vessel wall along their lengths and potentially become endothelialized therein. This condition is illustrated in prior art FIG. 1 in which a prior art filter 13 has been delivered through a delivery sheath 25 into a blood vessel 51. In the event of this occurrence, there is a greater likelihood of endotheliosis of the filter to the blood vessel along a substantial length of the filter wire. As a result, the filter becomes a permanent implant in a shorter time period than otherwise.
Some filters have been designed so that the filter has minimal contact with the vessel wall. Ideally, some filters can be removed after several weeks with minimal difficulty and little injury to the vessel wall. One such filter is described in U.S. Pat. No. 5,836,968. The filter is designed so that the filter wires or struts are not positioned parallel to the vessel walls or not in contact with the vessel walls for a substantial portion of the length of the filters. The ends of the struts contact the vessel walls and provide anchoring to reduce the likelihood of filter migration. When the filter is removed, a wire is docked to one end of the device while a sheath or sleeve is passed over the wire. Using counter traction by pulling the wire while pushing the sheath, the sheath is passed over the filter and the filter struts are retracted from the vessel wall. In this way, only small point lesions are created where the filter was attached to the vessel wall.
The filter of U.S. Pat. No. 5,836,968 teaches two levels of oppositely expanding filter wires or struts to insure that the filter is properly aligned in the lumen of the vessel. If the filter tilts or becomes misaligned with the central axis of the vessel, the filter wires will contact the wall of the vessel along a greater area, and become endothelialized. As a result of the two levels, removal of the filter from the blood vessel becomes impossible or at least difficult.
Additionally, the configuration of the second level of filter wires in the device of U.S. Pat. No. 5,836,968 provides a filter which may be too long for the segment of the vessel that the filter would normally be placed. The normal placement segment of a vena cava filter is between the femoral veins and the renal veins. If the lower part of the filter extends into the femoral veins, filtering effectiveness will be compromised. Moreover, it is not desirable to have filter wires crossing the origin of the renal veins, since the filter wires may interfere with the flow of blood from the kidneys. In the device disclosed in U.S. Pat. No. 5,836,968, both levels of filter wires are attached at one point as a bundle at the central axis of the filter. The resulting diameter of this bundle of filter wires results in a filter that may be too large for easy placement and becomes an obstacle to blood flow in the vena cava.